A novel strategy to assemble colloidal gold nanoparticles at the water–air interface by the vapor of formic acid

We report a novel strategy on the controlled assembly of gold nanoparticles (NPs) at the air–water interface by designing a concentration gradient of electrolytes utilizing volatile weak acidic electrolytes. Films of close-packed Au NPs can be facilely obtained by exposing citrate-protected gold colloids to the vapor of formic acid for several hours in an airtight desiccator at room temperature. Both the higher interfacial concentration of formic acid and the buffer effect of citrate solution play the key roles in the assembly. They engender a gradient distribution of hydrogen ions such that to trigger the interfacial assembly of gold NPs while preventing the bulk colloid from aggregation and coagulation. Comparative investigations have also been performed either using other volatile electrolytes like weaker acetic acid and stronger hydrochloric acid or adding an electrolyte directly into the colloids. The as-prepared films of gold NPs can serve as good substrates for surface-enhanced Raman scattering (SERS). This strategy has also been applied to the assembly of some other NPs like colloidal Pt at the air–water interface.

Graphical abstractA film of close-packed colloidal Au or Pt nanoparticles was obtained at the air–water interface by designing a concentration gradient of formic acid using its vapor.Figure optionsDownload full-size imageDownload high-quality image (32 K)Download as PowerPoint slideHighlights► A novel strategy of designing concentration gradient of formic acid in a gold colloid. ► Exposing a gold colloid to the vapor of formic acid to obtain close-packed films. ► Assembly at the air–water interface in short time without aggregation, coalesce or coagulation. ► Applicable to the interfacial assembly of other colloids such as platinum colloid. ► High SERS effect of the assembled film of gold nanoparticles due to their nanogaps.